Process for producing sensors for measuring spatial forces and sensors obtained

ABSTRACT

The invention relates to a sensor for measuring three components of force and three components of moment. This sensor is comprised of one unitary mechanical piece comprising two end faces (1, 2) able to be secured respectively to a body generating forces and moments and to a reference body, and a central portion (7) comprising six beams (8) provided with means (12) for measuring deformations, arranged according to a closed triangular architecture between the two end faces (1, 2).

The present invention relates to a process for producing sensors formeasuring spatial forces, that is, rigid mechanical structures which,when they are interposed between two bodies, permit measurement of thetorque between these two bodies, the torque being comprised of threecomponents of force and three components of moment exerted by one bodyon another. It also relates to sensors obtained by carrying out thisprocess.

The invention finds application in the fields of robotics, machine toolsand in a general manner, in any industry when measuring forces andcouples transmitted between two pieces.

Principal known apparatus of this type comprises several piecesassembled together by securing members, the assembly bordering on acomplex mechanical structure and requiring high quality manufacturing.

Such sensors are particularly described in patents SU 1,185,128, FR2,529,333 and JP -A- 5263827 providing wrist bands for the detection ofsix components of force, and patents EP 0,156,073 and U.S. Pat. No.4,196,337 providing cuffs for the detection of three force components.

To overcome these disadvantages, another process comprises providingsensors for measuring three components of torque as described in U.S.Pat. No. 4,094,192, comprising three beams and produced by removal ofmaterial from a single starting piece. However, this type of process hasbeen limited, as described in the preamble of U.S. Pat. No. 4,196,337,to sensors provided with three or four beams.

The present invention provides a sensor of this type produced by removalof material from a single starting piece and has as an essential objectto provide a sensor for measuring three components of force and threecomponents of moment, permitting providing very homogeneous measurements

Another object is to provide a very compact sensor permitting measuringvery high forces.

To this end, the invention relates to a process for production of asensor for measuring three components of force and three components ofmoment, characterized in that it comprises:

machining, by removal of material, a mechanical piece having the shapeof a sleeve of a generally rounded transverse shape, provided with aperipheral wall of a given thickness, in such a manner as to obtain,firstly, a sleeve having on its greatest length, a central portion of athickness less that the initial thickness, and two end portions of thesame thickness as the initial thickness,

secondly, hollowing out the central portion in such a manner as toprovide six beams connecting the end portions and having longitudinallya helical shape, said beams being arranged according to a closed,triangular architecture, and

providing each of the beams with means for measuring their deformation.

The invention thus permits provision of a sensor from a single startingpiece, by removals of material from predetermined places of this piece.This process therefor does not resemble any assemblage and permits alsothe provision of the sensors of various shapes, as functions of thecross-section of the single starting piece: circular, hexagonal, . . .

In addition to its economic interest, such a process permits obtaining asensor having fundamental advantages during use. In effect, the sensorproduced has great rigidity enabling measurement of high forces.Further, the beams being precisely set in place and the sensor beingproduced from a single piece of material, permits carrying out veryhomogeneous measurements.

Further, the helicoidal form of the beams permits providing a sensorhaving a better sensitivity, since due to their curvature, the beams aresubjected not only to tensile-compressive stresses, but also to flexuralstresses.

As a function of the applications thereof, the sensor may be producedeither from a straight casting or from a casting which is longitudinallybent in such a manner as to obtain parallel end portions or extendingalong secant planes.

The invention also relates to a sensor obtained by carrying out thisprocess and is characterized in that it comprises a one piece mechanicalelement having two end faces adapted to be secured respectively to abody generating forces and moments, and to a reference body, and acentral portion composed of six beams provided with means for measuringdeformations, and arranged according to a closed triangular architecturebetween the two end faces.

The thus sensor comprises a central part composed of six beams arrangedaccording to a triangular architecture and two end faces comprisingmeans for securing to the force and moment generating body and to thereference body. Thus, it constitutes a single mechanical piece, theprovision of which is not made apparent from any assemblage of parts.

During use of this sensor, one end face thereof is fixed to a bodygenerating forces and moments, and the other end face to a referencebody. The three components of forces and of moments are thus transmittedfrom one body toward the other through the sensor, and in particularthrough its central framework portion.

The application of the three components of force and moment to one endfact of the sensor generates, because of the triangular arrangement ofthe beams, forces being exerted at the level of each of the connectionsto the housing of each beam. These forces, having the same intensity,and different directions, cause a deformation of the beams.

Measuring means such as strain gauges fixed in the central region ofeach beam, deliver an electric signal as a function of the deformationof the beam.

The adaptation of this sensor to a given application is achieved by anappropriate choice of dimensions of the sensor body as well as its crosssection, the length and inclination of the beams forming the centralframework.

The mechanical structure of this sensor being rigid, it can measure highforces.

Other characteristics, objects and advantages of the invention willbecome apparent from the detailed description which follows and from aconsideration of the accompanying drawings which show, by way ofnon-limiting example, one preferred embodiment. In these drawings whichform an integral part of the present description:

FIG. 1 is a perspective view of the sensor;

FIG. 2 is an expanded plan view of the central portion of the sensor;

FIG. 3 shows a block diagram of the measurements provided from the sixbeams of the sensor;

FIG. 4 is a partial perspective view on an enlarged scale of the centralportion of the sensor.

The sensor shown in FIG. 1 is provided with two end faces 1 and 2 havingthe shape of rings 5, 6 permitting securing the sensor to bodies betweenwhich are to be measured the components of force and moment. These twoend faces 1, 2 are provided with securing means, in the example holes 3and 4, arranged axially in the rings 5, 6.

The central portion of the sensor, in a cylindrical shape, presentsitself in the shape of a spatial framework 7 formed of beams 8 securedby connections to the housing 10, 11 to the end faces 1, 2 and having ahelical shape in the longitudinal direction. These beams 8 are producedby the removal of material in the places 9 of this central portion ofthe sensor. They are distributed uniformly about the primary axis of thesensor and arranged according to a closed, angular architecture in sucha manner as to extend from three distinct zones from one of the endfaces 1 of the sensor to three distinct zones of the other end face 2spaced axially by 60 with respect to the three zones.

The components of force and moment exerted on one end face 1, 2 of thesensor generate opposing forces between the two connection zones 10, 11of each beam 8 and as a result deform the same.

The deformation is measured on each beam 8 by a strain gauge 12 whichdelivers an electric signal as a function of the deformation of thebeam.

Because of the framework structure of the sensor, the followingrelationship may be written between the components of torque to bemeasured (Fx, Fy, Fz, Mx, My, Mz) and the forces exerted on the beams 8of the sensor (F1, F2, F3, F4, F5, F6): ##EQU1##

The forces F1 to F6 generate a deformation of the beams 8: ε1, ε2, ε3,ε4, ε5, ε6 which are measured by means of the deformation sensors 12.

The relationship between the forces F1 to F6 and the deformations ε1 toε6 is:

    εI=Ki Fi.

When the beams 8 are identical, the terms Ki are identical.

The function which relates the electric signals μ1 to μ6 which arereceived to the deformations ε1 to ε6 is:

    μi=Gi εi

When the gauges 12 are identical, the terms Gi are identical. As aresult, we have a relationship between the signals which are receivedand the forces in the beams:

    μi=Gi×Ki×Fi

and: ##EQU2## and therefor the relationship between the signals whichare received and the torque to be measured will be: ##EQU3##

The terms q,i,j are identified by, for example, subjecting the sensor tounitray constraints according to each component.

When the sensor is placed on a robot, one end is fixed to a wrist of therobot, and the other end to the terminal.

One may then use the force data delivered by this sensor for controllingthe robot. The analog signals are converted to digital signals and sendto the computer controlling the robot (FIG. 3).

We claim:
 1. A process for the production of a sensor for measuringthree components of force and three components of moment,comprisingworking, by removal of material, a mechanical piece having theshape of a sleeve of a generally rounded cross-sectional shape and aperipheral wall of a given thickness, in such a manner as to obtainsubstantially simultaneously a sleeve having, over its greatest length,a central portion (7) of a thickness less than the initial thickness,and two end portions (1, 2) of the same thickness as the initialthickness, hollowing out said central portion (7) in such a manner as toproduce six beams (8) connecting said end portions (1, 2) and havinglongitudinally a helical shape, said beams being arranged according to aclosed triangular architecture, and providing each of the beams (8) withmeans (12) for measuring their deformation.
 2. A process for producing ameasuring sensor as in claim 1, and wherein said sleeve comprises arectilinear sleeve so as to obtain two parallel end portions (1, 2). 3.A sensor for measuring three components of force and three components ofmoment comprising a unitary mechanical piece having a generally roundedtransverse cross-section and comprising two end faces (1, 2) able to besecured respectively to a body generating forces and moments and to areference body, and a central portion (7) composed of six beams (8)having longitudinally a helical shape, provided with means (12) formeasuring the deformations and arranged according to a closed triangulararchitecture between the two end faces (1, 2).
 4. A measuring sensor asin claim 3, and wherein said end faces (1, 2) are parallel.
 5. A sensoras in one of claim 3, in which the beams (8) define a regular frameworkand extend from three distinct zones of one of the end faces (1) of thesensor to three distinct zones of the other end face (2), spaced axiallyby 60° with respect to the aforesaid three zones.
 6. A sensor as in oneof claim 3, in which the means for measuring the deformation of thebeams (8) comprises strain gauges (12).